There is a growing interest in the printing industry for digital printing of inks onto containers and labels. Typical digital printing currently employed includes gravure and flexographic printing. However, such printing processes have the disadvantage of an arduous set up which means that the economic running time limits the possibility of short runs and limits customisation of the printed products.
Inkjet printing is an ideal process in order to overcome such issues on account of its flexibility and ease of use. However, inks, which are suitable for use in inkjet printing, can cause problems when applied to a food packaging substrate.
In this respect, food packaging represents a particular challenge on account of the strict safety limitations on the properties of materials, which come into contact with food, including indirect additives like packaging inks. For printed food packaging, it is necessary to control and quantify the migration and/or odour of the components of the printed image on the food packaging into the food products. Specific exclusions based on their odour and/or migration properties include volatile organic solvents and many monomers typically used in UV curing inks.
Printing using UV curing inks, which comprise reactive monomers, is dependent on the dose of UV radiation directed at the printed film to effect polymerisation of the reactive components in the ink. This has a major influence on the levels of unreacted residual monomer.
In gravure and flexographic printing, the issues of migration of components and malodour of UV curing inks in food packaging are overcome by formulating inks having (meth)acrylate monomers with at least three (meth)acrylate groups per molecule. This high level of (meth)acrylate functionality maximises the likelihood that the monomer will be incorporated into a growing polymer chain, thus making it unavailable to migrate into the foodstuff. This is not possible for inkjet inks as the higher functionality monomers cannot be used as the sole monomer in UV inkjet inks owing to their high viscosity. In this respect, inks suitable for inkjet printing typically need a low viscosity, preferably below 25 mPas at jetting temperature. It is thus necessary to blend the inks with lower viscosity materials, which can compromise the migration/odour characteristics of the printed film.
A potential solution to this problem is to include a solvent into the ink composition to reduce the viscosity, as disclosed in WO 2013/093414. WO 2013/093414 discloses compositions comprising high molecular weight (greater than 1,000 Daltons) multifunctional (meth)acrylate oligomers diluted with an organic solvent. Materials with molecular weights of over 1,000 Daltons are widely recognised as being too large to pass through cell membranes and hence it is desirable to use such components in inks for use in the food packaging industry. However, although this is solution to the problem of controlling the potential for migration of (meth)acrylate materials into foodstuffs, it is essential that the organic solvent is fully removed during the drying and curing process. This is particularly problematic when using absorbent substrates. Of course, the use of a low toxicity alternative, namely water, as opposed to the organic solvent solves this issue. However, water-compatible (meth)acrylate functionalised radiation-curable material having a molecular weight of greater than 1,000 Daltons is required. The material also needs to yield films with good resistance properties and allow formulation of low viscosity compositions. This is problematic in aqueous systems where hydrogen bonding can give rise to high viscosity solutions even with relatively low molecular weight species.
There is therefore a need in the art for inkjet inks which can be printed directly onto food packaging, have low odour and migration properties, the required viscosity, without compromising the physical film properties required, such as chemical and scratch resistance.